At present, as a headlamp of a vehicle, a short arc type high-pressure discharge metal halide lamp is coming into use. A metal halide lamp has a structure in which a pair of electrodes are disposed facing each other inside an arc tube having a metal halide and a rare gas sealed therein.
In this metal halide lamp, a radioactive material is sometimes used for suppressing flickering. For example, as the metal halide, thorium is sealed in a discharge space or thorium oxide is mixed in an electrode. However, since thorium is a substance of concern, it is desired to avoid using thorium, and thus, it is required to avoid using thorium, in other words, it is required to form a thorium-free lamp. Further, recently, there is a demand for power saving, and a low-power lamp whose lamp power is reduced to 25 W from a conventional lamp power of 35 W is proposed.
In this manner, as the lamp to be used for a vehicle, a low-power and thorium-free lamp is demanded. However, it was found that in such a lamp, not only flickering is liable to occur since thorium is not used, but also electrodes are liable to be deformed.